This invention relates to a process for producing smokeless, cured fuel briquettes from particles of combustible solid carbonaceous material, in particular coal particles, such as coal fines, anthracite duff, etc.
Many briquetting methods are known that agglomerate particulate carbonaceous materials using bonding agents in briquetting machines. However, the agglomerates thus obtained, called "green briquettes", often require a curing treatment in order to improve their physical and/or chemical characteristics. In addition, these green briquettes often require a desmoking treatment in order to reduce the evolution of smoke during their combustion. Smokeless briquettes are defined hereinafter according to British Standard 3841.
Current agglomeration technology for producing coal briquettes is generally based upon the use of three principal binders, either separately or together, although many other binders are known in the art. These three binders are bitumen derived from the refining of crude oil; coal tar pitch; and ammonium lignosulfonate or sulfite waste liquor which is a by-product from the paper industry. The use of bitumen or coal tar pitch as a binder is a well-established process practiced by several manufacturers and is generally associated with an oven cure technique in which the green briquettes are desmoked at medium temperatures in an oxidizing atmosphere.
The use of ammonium lignosulfonate as a binder for carbonaceous briquettes used in the smokeless fuel market is not as widely used as bitumen or coal tar pitch. However, processes are known in the art that use ammonium lignosulfonate as a binder. These processes generally incorporate an oven cure technique in the presence of an oxygen containing atmosphere in which the oxygen content approaches the oxidizing stoichiometric amount or near reducing conditions. This limitation of oxygen content was necessary when using this binder because there was a need to control or limit the possibility of rapid oxidation and exothermic reactions. If the oxygen content was not limited, the rapid oxidation and exothermic reactions would lead to uncontrollable combustion of the briquettes during treatment and consequent loss and damage to product and plant. However, this necessary limitation of oxygen content during the curing treatment of agglomerates leads to several drawbacks, the major drawback being the production of undesirable gases. When working under a near reducing atmosphere, the sulfur of the lignosulfonate is transformed into mercaptans, hydrogen sulfide and other noxious and toxic compounds that present a pollution problem. Moreover, the curing rate is too low and the cured briquettes have a poor physical strength.